Restricted Visibility

Entering Restricted Visibility

When restricted visibility is encountered or expected, the first responsibility of the officer of the watch is to comply with the 1972 International Regulations for Preventing Collisions at Sea (COLREGS) and the Master’s Standing Orders, particularly:

The times of entering and leaving restricted visibility should be noted in the logbook as well as any actions taken in accordance with the above list.

Collision Avoidance in Restricted Visibility

In conditions of restricted visibility when complying with Rule 19; there are no stand-on or give-way vessels. All vessels are required to determine whether a close-quarters situation is developing and if a risk of collision exists. If the likelihood of a close-quarters situation is detected then each vessel must take appropriate action to prevent the close-quarters situation from developing. 

The judgment as to when a vessel is heading in to a close-quarters situation will have to be made by the officer of the watch using all available means combined with their own experience and good seamanship. However, this should be discussed fully with the Master when they are on the bridge. Over-reliance must not be placed on calculated CPAs and TCPAs from the ARPA equipment. Navigational constraints, environmental factors and knowledge of own vessels manoeuvrability must also be taken into account when reaching this judgement. 

When taking action to avoid collision in restricted visibility, it should be remembered that the terms of rule 8 still apply. The vessel should make a single alteration of course or speed that is readily apparent to the other vessel (should they be monitoring), made in good time and result in passing at a safe distance. In deciding on the alteration to make the following should be remembered:

• The ship should avoid an alteration of course to port for a vessel forward of her beam, except for a vessel being overtaken.
• The ship should avoid an alteration of course towards a vessel abeam or abaft her beam.

Any OOW should be encouraged to call the Master or seek the view of more experienced members of the bridge team in ample time to assist in assessing the situation and determining the best action to take. Use of trial manoeuvre or equivalent practical plotting techniques should be used effectively to ensure that any alteration does not result in another close-quarters situation developing.

Radars and Restricted Visibility

When entering rain, the officer of the watch should be aware of the effects of radar propagation. Certain radio wavelengths will be attenuated more than others by rain and therefore the officer of the watch should use the correct radar for the situation that presents itself.

When approaching rain, the X-band, 3 cm radar will be more susceptible to reflection and attention from the rain. That is because the smaller wavelengths are much more liable to poor propagation due to falling rain. This will not be solved by addressing the rain or sea clutter settings, as it is due to the nature of the wave not the reception of the radar.

The officer of the watch should switch over to the 10 cm, S-band radar when navigating in or near falling rain. The longer wavelengths are able to pass relatively unhindered through the rain and therefore will provide a detailed picture of the surrounding situation when directly compared with an X-band. The S-band radar is fitted with all of the same programs as the X-band, ARPA, parallel index lines, etc and so can be used exactly like the X-band. The reason for not using it all of the time is that the 10 cm wavelength does lead to a slight loss of resolution when compared to the 3 cm which can make navigation slightly less accurate.